Why was the little boy crying?
Because he had a frog stapled to his forehead.

My informant told me this joke after I had been venting about how bad my day had been.  At this point, she told me this joke.  After she delivered the punchline, I was at first taken aback by how violent the answer was. But then, I started to laugh at how far out and unexpected it was.  I asked her where she heard this joke, and she told me that she had heard it from school.

This joke is an example of an anti-joke.  An anti-joke is characterized by having the question be one that can have many different answers to it.  Another characteristic is that the punchline is usually not funny.  However, the unexpected nature of the punchline is really what makes the joke humorous.  This form of jokes probably developed as a way to add a twist to the stereotypical jokes that are out there.